1. Field of the Invention
The present invention relates in general to an in-plane switching liquid crystal display (IPS-LCD). In particular, the present invention relates to an IPS-LCD with a compensation electrode structure and a method of forming the same.
2. Description of the Related Art
Liquid crystal displays (LCDs) may be classified by the orientation of the liquid crystal molecules between the spaced apart glass substrates. In a conventional twisted nematic LCD (TN-LCD), the liquid crystal molecules are twisted between the two substrates. In contrast, in an in-plane switching LCD (IPS-LCD), common electrodes and pixel electrodes are formed on a lower glass substrate (TFT substrate) and an in-plane electric field therebetween is generated to rearrange the liquid crystal molecules along the electric field. Accordingly, the IPS-LCD has been used or suggested for improving drawbacks of the conventional TN-LCD, such as a-very narrow viewing angle and a low contrast ratio.
In order to achieve a better result of the in-plane electric field, a comb-shaped electrode array is built in the IPS-LCD to solve the problems such as an insufficient aperture ratio and crosstalk produced between data lines and common electrodes. FIGS. 1A and 1B are sectional diagrams of a conventional IPS-LCD, and FIG. 1C is a top view showing an electrode array within a pixel area of an IPS-LCD according to the prior art. FIG. 1A shows the alignment of the liquid crystal molecules at an off state, and FIG. 1B shows the alignment of the liquid crystal molecules at an on state. The IPS-LCD has a lower glass substrate 10, an upper glass substrate 12, and a liquid crystal layer 14 disposed in a space between the two parallel glass substrates 10 and 12. On the lower glass substrate 10, serving as a TFT substrate, a plurality of strip-shaped common electrodes 16 arranged as a comb-shape structure is patterned on the lower glass substrate 10, an insulating layer 18 is deposited on the common electrodes 16 and the lower glass substrate 10, and a plurality of strip-shaped pixel electrodes 20 arranged as a comb-shape structure is patterned on the insulating layer 18.
As shown in FIG. 1A, before an external voltage is applied to the IPS-LCD, the liquid crystal molecules 14A are aligned in a direction parallel to the lower glass substrate 10. As shown in FIG. 1B, when an external voltage is applied to the IPS-LCD, an in-plain electric field is generated between the common electrode 16 and the pixel electrode 20, resulting in a rotation of the liquid crystal molecules 14B toward the in-plane electric field.
Depending on the material and the structure design of the common electrode 16 and the pixel electrode 20, the conventional comb-shaped electrode array is classified as three types. FIGS. 2A to 2C are sectional diagrams showing three types of the common electrode 16 and the pixel electrode 20 in the conventional comb-shaped electrode array. In the first type, as shown in FIG. 2A, the common electrode 16 and the pixel electrode 20 are patterned on the same plane and made of a transparent conductive material, such as ITO or IZO. In the second type, as shown in FIG. 2B, the common electrode 16 made of a non-transparent conductive material, such as Al and MoW, is patterned on the lower glass substrate 10 followed by depositing the insulating layer 18, and then the pixel electrode 20 made of a transparent conductive material, such as ITO or IZO, is patterned on the insulating layer 18. In the third type, as shown in FIG. 2C, the common electrode 16 and the pixel electrode 20 are patterned on the same plane and made of a non-transparent conductive material, such as Al and MoW. By comparison, the first type shown in FIG. 2A can provide an greater luminance to the IPS-LCD than the second type shown in FIG. 2B and the third type shown in FIG. 2C, but provides a worsen view-angle characteristic than the second type and the third type. Also, the third type severely decreases the luminance of the IPS-LCD because most of the light is blocked by the non-transparent conductive material. Therefore, the second type shown in FIG. 2B is the most common type used in the conventional comb-shaped electrode array.
However, as to the second type, since the common electrode 16 and the pixel electrode 20 are patterned on different planes, it is possible to form different intervals between the common electrodes 16 and the pixel electrodes 20 on the electrode array caused by misalignment in the photolithography process. FIG. 3A is a sectional diagram showing an ideal case when a constant interval is formed between the common electrode 16 and the pixel electrode 20. FIG. 3B is a transmittance-position diagram according to the electrode array shown in FIG. 3A. FIG. 4A is a sectional diagram showing a practical case when different intervals are formed between the common electrodes 16 and the pixel electrodes 20. FIG. 4B is a transmittance-position diagram according to the electrode array shown in FIG. 4A. As shown in FIGS. 3A and 3B, the interval between the common electrode 16 and the pixel electrode 20 is a constant S1, and each S1 spacing region has the same degree of in-plane electric field, resulting in the same capacitance and transmittance. In contrast, as shown in FIGS. 4A and 4B, the intervals between the common electrode 16 and the pixel electrode 20 are different, such as S1 and S2, and the S1 spacing region and the S2 spacing region have different degrees of in-plane electric field, resulting in different capacitances and transmittances. In this practical case, demerits such as trip mura, shot mura and flicker are commonly found in the conventional IPS-LCD.